1. Field of the Invention
The present invention relates generally to a mask for making polysilicon, a method of making the same and a method of making thin film transistor using the same, and more particularly to a mask for crystallizing amorphous silicon into polysilicon, a method of making the same and a method of making thin film transistor using the same.
2. Description of the Related Art
A liquid crystal display (LCD) includes a couple of substrates having electrodes and a liquid crystal layer sandwiched between the substrates. The substrates are attached to each other by a sealant that is usually printed along the edges of the substrates. When attached to each other, the substrates enclose the liquid crystal layer.
An electric field is applied to the liquid crystal layer through the electrodes. The liquid crystal layer has an anisotropic dielectric constant, and images can be displayed by adjusting the transmittance of light through the liquid crystal layer by changing the electric field. A thin film transistor (TFT) is used to control the signals to the electrodes.
Generally, amorphous silicon is used as a channel area of the TFT. The mobility of amorphous silicon is about 0.5˜1 cm3/Vsec. While this mobility range is acceptable for use in a switching device of the LCD, it is not acceptable in a drive circuit formed directly on a liquid crystal panel.
To overcome this problem with amorphous silicon, a polysilicon TFT having a channel area that is formed with a polysilicon has been developed. Polysilicon has a mobility of about 20˜150 cm3/Vsec, and this higher mobility of the polysilicon TFT enables the drive circuit to be formed directly on the liquid crystal panel, resulting in the so-called chip-in-glass configuration.
As methods of making a thin film of polysilicon, there are methods of depositing polysilicon directly on the substrate at high temperature, crystallizing a deposited amorphous silicon at high temperature of about 600° C., and crystallizing a deposited amorphous silicon by heat treatment using laser. However as each of these methods involve a high temperature process, use of a glass substrate is difficult. Further, polysilicon formed by these methods has an uneven grain boundary, resulting in inconsistent electrical property of the TFT.
Accordingly, sequential lateral solidification (SLS) method, which can adjust the distribution of the grain boundary, has been developed to solve the above problem. The SLS method uses the fact that when laser is irradiated to a part of amorphous silicon, thereby partly melting the amorphous silicon, the grain of the polysilicon grows from the boundary between a solid area that is not irradiated by the laser and a liquid area that is irradiated by the laser. The growth direction of the grain is generally perpendicular to the boundary.
In the SLS method, the laser beam is irradiated through a transmitting area of a mask, forming a liquid area of molten amorphous silicon layer that approximately matches the shape of the transmitting area. The transmitting area of the mask may include a slit. The grains of the polysilicon grow as described above and the growth of the grains is complete when the grains growing from opposite boundaries meet each other at or near the center of the liquid area. The adjacent amorphous silicon layer is crystallized by moving the mask in the direction of grain growth intermittently, wherein the size of the grain corresponds to the width of the slit.
The mask used in the SLS method conventionally comprises a quartz substrate and a chrome pattern formed thereon. The mask has a blocking area where the irradiation of the laser to the amorphous silicon is blocked and a transmitting area where the laser is irradiated to the amorphous silicon. The chrome pattern is formed at the blocking area.
As chrome has high heat absorption rate and the laser used in the SLS method has considerable energy, the chrome pattern of the mask is easily overheated. Further, as laser is irradiated intermittently, the chrome pattern undergoes a repeated cycle of heating and cooling. This heating and cooling cycle applies thermal stress on the chrome pattern. This thermal stress is increased by the difference in heat expansion coefficient between the chrome and the quartz. After tens of millions of laser shots, the chrome pattern becomes deformed, necessitating mask replacement. This need for periodic mask replacement undesirably increases the production cost.
A mask that does not suffer from the above problems, and therefore having a longer life span, is desired.